1. Field of the Invention
This invention relates to solid electrolytic capacitors having good capacitor characteristics, particularly good high frequency characteristics and high reliability under high temperature and high humidity conditions. The invention also relates to a method for manufacturing such capacitors.
2. Description of the Prior Art
In recent years, digitalization in the electric and electronic fields has been conspicuous. This entails a strong demand of capacitors which exhibit a low impedance in a high frequency range and are small in size and large in capacitance.
Known capacitors which are used in high frequency ranges include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. These capacitors are disadvantageously so large in size that difficulties are involved in obtaining a large capacitance.
On the other hand, a certain type of capacitor is known as having a large capacitance. Such a capacitor includes, for example, an aluminium dry electrolytic capacitor and an aluminium or tantalum solid electrolytic capacitor. These electrolytic capacitors are advantageous in that since an anodized film serving as a dielectric layer can be formed very thinly, a large capacitance can be realized. However, the anodized film is liable to damage, so that it becomes necessary to provide an electrolyte between the anodized film and a cathode in order to repair the defects. With aluminium dry electrolytic capacitors, anode and cathode aluminium foils which have been etched, respectively, are convolutely wound through a separator and a liquid electrolyte is impregnated in the separator. This presents the problems that since the liquid electrolyte is ion conductive in nature with a large specific resistance, so that the loss (tan .delta.) is great with very poor frequency and temperature characteristics and that occurrence of the leakage and evaporation of the liquid electrolyte is inevitable, leading to a decrease of the capacitance and an increase of the loss with time. With the tantalum solid electrolytic capacitor, manganese dioxide is used as the electrolyte, the problems on the temperature characteristic and the changes of the capacitance and loss in relation to the time can be overcome. However, the relatively high specific resistance of manganese dioxide results in a frequency characteristic poorer than those of the layer-built ceramic capacitor and film capacitor.
In order to solve the above problems, there has now been proposed use of organic semiconductors having good anodizability such as 7,7,8,8-tetracyanoquinodimethane complexes (hereinafter referred to simply as TCNQ complex or complexes). As set forth in Japanese Patent No. 56-10777 and Japanese Kokai No. 58-17609, the aluminium solid electrolytic capacitor using the TCNQ complex is remarkably improved in the frequency and temperature characteristics with a low leakage current characteristic being achieved. Since the TCNQ complex is thermally stable among organic conductive materials, the life of the capacitor at high temperatures is significantly superior to those of known dry electrolytic capacitors.
Further, there have been recently proposed solid electrolytic capacitors wherein highly conductive polymers, which contain an anion of a support electrolyte as a dopant and which are obtained by electrolytically polymerizing heterocyclic monomers such as pyrrole, thiophene and the like, are formed on an anode for use as an electrolyte (Japanese Kokai Patent Application Nos. 60-37114 and 60-244017).
According to the electrolytic polymerization, a conductive polymer film may be formed on an anode by electrolytic oxidation, but the formation of the conductive polymer on an anode having a dielectric oxide film by the electrolytic polymerization is difficult without breakage of the dielectric film. Even if the electrolytic polymerization will be performed while partially breaking the dielectric oxide film, the growing rate of the polymer will become be so slow that it takes a long time before coverage of the entire surface with the polymer film. If the polymer film formed by the electrolytic polymerization is grown from a broken portion of the dielectric oxide film on an anode whose surface area is increased such as by etching, the growth does not reach the inside of etch pits. This makes it difficult to obtain a capacitor which has a large capacitance.
The valve metal which is not subjected to formation of a dielectric film may be covered directly with the polymer formed by the electrolytic polymerization. After the covering, the dielectric oxide film may be formed through anodization. In this case, however, the anodization reaction has to be carried out through the electrolytically polymerized film. This will cause the polymerized film to be degraded or the adhesion to the valve metal surface to be lower. Thus, the resultant capacitor will not have good characteristics.
To avoid this, several attempts have been made. In one such attempt, an electrode for the polymerization is contacted from outside with a valve metal on which a dielectric oxide film has been formed, thereby forming an electrolytically polymerized film. In this case, the dielectric oxide film is liable to be damaged by the contact with the polymerization electrode, with the attendant problem that a leakage current characteristic and a breakdown voltage are lowered. Where the polymerization electrode is provided at a small distance from the valve metal, the conductive film starts to grow from the polymerization electrode and contacts with the valve metal, permitting the film to be formed over the valve metal surface. This is disadvantageous in that after completion of the electrolytic polymerization, the polymerization electrode has to be removed, whereupon part of the polymerized film is inevitably separated from the valve metal. The separation eventually leads to an increase of the leakage current and a lowering of the breakdown voltage.
In another attempt, a conductive layer is formed on a valve metal surface having an oxide film, with which a polymerization electrode is contacted from outside, thereby forming an electrolytically polymerized film through the conductive layer. Manganese oxide is usually used as the conductive layer for contact with the electrode since it has the repairing ability of the oxide film on the valve metal. When a capacitor is fabricated using a valve metal in the form of a sheet or foil, it is necessary to separately produce a capacitor region covered with an electrolyte and an anode terminal. The capacitor region covered with the electrolyte should be clearly separated from the other region so as to prevent a short-circuiting or leakage current failure. However, when the sheet or foil is immersed in a polymerization solution containing a monomer and a support electrolyte and subjected to electrolytic polymerization, the boundary between the conductive polymer film and the film-free region becomes unclear owing to vibrations occurring in the surface of the solution, the formation of meniscus and the capillary action of the solution. Accordingly, there is the likelihood of short-circuiting being produced or a leakage current failure. To avoid this, a relatively large distance between the anode terminal and the capacitor region has to be provided, thus leading to a difficulty in enhancing a volumetric efficiency.
In addition, when a capacitor element which has a convolutely wound structure of an anode and a cathode through a separator like dry aluminium electrolytic capacitors is fabricated by the electrolytic polymerization technique, satisfactory diffusion of a monomer and ions of a support electrolyte is not possible with the difficulty in uniform formation of the conductive polymer film throughout the capacitor region. In general, the conductive polymer film obtained by the electrolytic polymerization is rigid and is difficult to mechanically process such as bending after the formation of the film. Accordingly, the capacitor region has to be used in the form of a flat sheet or in the form of a convolutely wound structure which has a relatively large curvature.